Many spray painting machines have been introduced to paint mass-produced items such as automotive bodies. Such paint spraying machines have typically been limited in their use in that they must be backed up by human operators, who must touch up areas missed by the machines. Such machines also tend to be wasteful of paint and are only designed to paint with horizontal and/or vertical paths on a reciprocator system. Coating an object fully, especially if it has complex contours, requires movements in depth as well as lateral movements. The use of a wrist is significant in that it is often necessary for an operator to angle a paint spray gun in a particular manner to deposit paint on a heavily contoured surface.
Many of the prior art spray painting machines include a battery of spray guns fed from large capacity, centralized paint reservoirs which manage to paint a large percentage of the exterior surface to be covered. Less accessible areas, such as wheel arches, the interiors of the trunk or engine compartment and door edges had to be painted by operators who looked for unpainted areas as the car body left the automatic painter.
Numerous prior art patents disclose painting machines including electrical painting apparatus such as the Chapman U.S. Pat. No. 2,858,947; the Shelley, et al U.S. Pat. No. 3,007,097; the Pierson et al U.S. Pat. No. 3,481,499; the Richter U.S. Pat. No. 4,030,617; the Yoshio U.S. Pat. No. 4,113,115; the Burns et al U.S. Pat. No. 4,196,049; the Shum U.S. Pat. Nos. 4,398,863 and 4,407,625; the Jacot-Descombes et al U.S. Pat. No. 4,424,472 and the Gorman U.S. Pat. No. 4,424,473. The Pollard U.S. Pat. Nos. 2,213,108 and 2,286,571 both disclose electrical robots for paint spraying. The Stricker U.S. Pat. No. 4,170,751 likewise discloses an electric apparatus associated with paint spraying robots.
There are many reasons for using spraying robots to provide high-quality painted finishes on a mass-produced item. Robots are desirable in that they are able to cope with the hostile painting environment; they allow the painting process to proceed with less total energy being expended; and they improve paint quality which, in turn, will eventually result in reduced material and labor costs. The above advantages are particularly important in painting car bodies where production rates are expected to be high, and there is a relatively small amount of time available for the paint to be applied and completed in not one, but several coats.
A spraying robot can also be used in the enameling and/or powdering of surfaces. For example, application of vitreous enamel to plastic bathtubs utilizes many of the same spray techniques as used in the application of paint to an automotive body.
Robots which have been used for spray painting comprise continuous path machines which emulate the action of a human operator. Such robots are typically taught by having an expert painter lead each robot in its learning mode through an actual paint job.
The average spray booth in the automotive industry has been standardized to have a relatively small width. This small width restricts the size and movements of any robot which is to be used in such a spray booth. Redesigning the existing spray booth is cost prohibitive.
Because of the limited size available for a robot in a conventional paint spray booth, the use of a robot having electrical drives is, at first blush, desirable due to the cost advantage that small electrical robots enjoy over small hydraulically-driven robots. Such costs not only include the cost of the robot, but also installation, maintenance and other operational expenses. However, in paint spraying and other similar applications the environment favors the use of hydraulically or pneumatically driven equipment. Such environment presents an explosion hazard to electrical motors and the electrical robot must either be explosion proof or intrinsically safe so as not to ignite the combustible environment. A hydraulically-driven robot does not utilize the amount of electrical energy typically sufficient to ignite the explosive fuel-air mixture.
Electrical equipment which is to be located in areas classified as "hazardous" (i.e. a Class 1, Division 1 location) by Article 500 of NFPA 70, Natural Electrical Code, either must be placed in pressurized containers or must be made explosion proof. If this is done the area immediately around the electrical equipment is no longer classified as a Class 1, Division 1 location, but rather a Class 1, Division 2 location wherein only the location adjacent the enclosure or explosion proof container contains the ignitable concentration of flammable gases or vapors under normal operating conditions. The pressurization of the enclosure entails supplying the enclosure with clean air or an inert gas with or without continuous flow at sufficient pressure to prevent the entrance of combustible gases or vapors which might occasionally be communicated into the enclosure. If the enclosure is maintained under a positive pressure of at least 25 pascals (0.1 inches of water) when the electric equipment is energized, the risk of an explosion in the "hazardous" environment is substantially eliminated.
One way of complying with the above standard is to make the containers for the equipment, such as motors, explosion-proof. However, the use of explosion-proof motors not only increases the cost of the motors, but also increases the weight and size of the robot. Also, the use of explosion-proof motors necessitates the use of explosion-proof cables. Such cables not only are more costly and heavier, but also are more inflexible and unwieldly. Such explosion-proof motors and cables also take up valuable space in or on the robot and, consequently, in the paint spray booth.